Terminal metal phosphinidene complexes (L n M = PR) are, despite continued interest, far less developed than their isolobal metal imide and alkylidene counterparts.[1] Although the first L n M=PR complex was reported over a quarter of a century ago, [2] sterically demanding R-groups are required, as M=PR linkages are reactive and require kinetic stabilization.[3] Certainly, free phosphinidenes (PR) are usually very reactive owing to their triplet ground states and unsaturated valence shells.[4] Although stabilization of a triplet PR group by a triplet metal fragment to generate a formal M=P bond is an attractive strategy, unlike the well-known L n M=NH and L n M=CH 2 linkages, [5] there has never been a structurally authenticated report of a d-/f-block metal-stabilized terminal parent phosphinidene L n M = PH, [6,7] and studies of such species are limited to computational investigations. [8] This paucity is underscored by a triplet-singlet energy gap of 22 kcal mol À1 for free PH, [4b] which has only been observed transiently in the gas phase or low temperature matrices. [4a, 9]

To further our fundamental understanding of the nature and extent of covalency in uranium-ligand bonding, and the benefits that this may have for the design of new ligands for nuclear waste separation, there is burgeoning interest in the nature of uranium complexes with soft- and multiple-bond-donor ligands. Despite this, there have so far been no examples of structurally authenticated molecular uranium-arsenic bonds under ambient conditions. Here, we report molecular uranium(IV)-arsenic complexes featuring formal single, double and triple U-As bonding interactions. Compound formulations are supported by a range of characterization techniques, and theoretical calculations suggest the presence of polarized covalent one-, two- and threefold bonding interactions between uranium and arsenic in parent arsenide [U-AsH2], terminal arsinidene [U=AsH] and arsenido [U≡AsK2] complexes, respectively. These studies inform our understanding of the bonding of actinides with soft donor ligands and may be of use in future ligand design in this area.

Determining the electronic structure of actinide complexes is intrinsically challenging because inter-electronic repulsion, crystal field, and spin–orbit coupling effects can be of similar magnitude. Moreover, such efforts have been hampered by the lack of structurally analogous families of complexes to study. Here we report an improved method to U≡N triple bonds, and assemble a family of uranium(V) nitrides. Along with an isoelectronic oxo, we quantify the electronic structure of this 5f1 family by magnetometry, optical and electron paramagnetic resonance (EPR) spectroscopies and modelling. Thus, we define the relative importance of the spin–orbit and crystal field interactions, and explain the experimentally observed different ground states. We find optical absorption linewidths give a potential tool to identify spin–orbit coupled states, and show measurement of UV···UV super-exchange coupling in dimers by EPR. We show that observed slow magnetic relaxation occurs via two-phonon processes, with no obvious correlation to the crystal field.

Carbon monoxide (CO) is in principle an excellent resource from which to produce industrial hydrocarbon feedstocks as alternatives to crude oil; however, CO has proven remarkably resistant to selective homologation, and the few complexes that can effect this transformation cannot be recycled because liberation of the homologated product destroys the complexes or they are substitutionally inert. Here, we show that under mild conditions a simple triamidoamine uranium(III) complex can reductively homologate CO and be recycled for reuse. Following treatment with organosilyl halides, bis(organosiloxy)acetylenes, which readily convert to furanones, are produced, and this was confirmed by the use of isotopically 13 C-labeled CO. The precursor to the triamido uranium(III) complex is formed concomitantly. These findings establish that, under appropriate conditions, uranium(III) can mediate a complete synthetic cycle for the homologation of CO to higher derivatives. This work may prove useful in spurring wider efforts in CO homologation, and the simplicity of this system suggests that catalytic CO functionalization may soon be within reach.reduction | C-C coupling | ethyne diolate | antiferromagnetic exchange coupling T he continued growth and stability of the global economy requires the ready availability of petrochemical feedstocks, but uncertainty in cost and supply, underscored by the energy crisis in the 1970s, has driven the demand for developing alternative sources (1). CO is readily produced by steam reforming reactions and is, thus, an abundant resource that can be used in the production of bulk hydrocarbon feedstocks (2). The concept of directly homologating CO is very appealing, but the triple bond in CO is very strong with a bond energy of ∼257 kcal mol −1 , and direct dimerization of CO to O ¼ C ¼ C ¼ O is highly unfavorable with ΔG°2 98 K ≈ þ73 kcal mol −1 (3). Although direct 1,1-migratory insertion of CO into an organometallic M-R bond to give M-C(O)R is a favorable process for many metals (4) and a key step in industrially important C-C bond formation reactions, e.g., hydroformylation (5), double insertion to give M-C(O)C(O)R is usually energetically unfeasible, but it has been documented (6-8). Thus, processes involving CO can require energy intensive reaction conditions and give varied product distributions that render them economically uncompetitive overall when crude oil remains readily available; however, combining the coupling of CO with reduction results in a thermodynamically feasible process to give a family of oxocarbon dianions C n O n 2− (n ¼ 2-6), including the reductively dimerized ethyne diolate − O-C ≡ C-O − , as building blocks for more complex and value-added organic molecules (9).Molten potassium can effect reductive oligomerization of CO, but the resulting ill-defined salts are thermally unstable and shock-sensitive (10). CO can be electrochemically homologated, but an overpressure of 100-400 bar of CO is required (11). Very few transition metal complexes mediate the reductive coupling...

The first structurally authenticated molecular uranium-transition metal bond is reported; DFT studies show sigma- and pi-components in the U-Re bond and this is the first time that the latter component has been reported in an unsupported f-element-transition metal bond.

We report on the role of 5f-orbital participation in the unexpected inversion of the s-bond metathesis reactivity trend of triamidoamine thorium(IV) and uranium(IV) alkyls. Reaction of KCH 2 Ph with [U(Tren TIPS)(I)] [2a, Tren TIPS ¼ N(CH 2 CH 2 NSiPr i 3) 3 3À ] gave the cyclometallate [U {N(CH 2 CH 2 NSiPr i 3) 2 (CH 2 CH 2 NSiPr i 2 C[H]MeCH 2)}] (3a) with the intermediate benzyl complex not observable. In contrast, when [Th(Tren TIPS)(I)] (2b) was treated with KCH 2 Ph, [Th(Tren TIPS)(CH 2 Ph)] (4) was isolated; which is notable as Tren N-silylalkyl metal alkyls tend to spontaneously cyclometallate. Thermolysis of 4 results in the extrusion of toluene and formation of the cyclometallate [Th {N(CH 2 CH 2 NSiPr i 3) 2 (CH 2 CH 2 NSiPr i 2 C[H]MeCH 2)}] (3b). This reactivity is the reverse of what would be predicted. Since the bonding of thorium is mainly electrostatic it would be predicted to undergo facile cyclometallation, whereas the more covalent uranium system might be expected to form an isolable benzyl intermediate. The thermolysis of 4 follows well-defined first order kinetics with an activation energy of 22.3 AE 0.1 kcal mol À1 , and Eyring analyses yields DH ‡ ¼ 21.7 AE 3.6 kcal mol À1 and DS ‡ ¼ À10.5 AE 3.1 cal K À1 mol À1 , which is consistent with a s-bond metathesis reaction. Computational examination of the reaction profile shows that the inversion of the reactivity trend can be attributed to the greater f-orbital participation of the bonding for uranium facilitating the s-bond metathesis transition state whereas for thorium the transition state is more ionic resulting in an isolable benzyl complex. The activation barriers are computed to be 19.0 and 22.2 kcal mol À1 for the uranium and thorium cases, respectively, and the latter agrees excellently with the experimental value. Reductive decomposition of "[U(Tren TIPS)(CH 2 Ph)]" to [U(Tren TIPS)] and bibenzyl followed by cyclometallation to give 3a with elimination of dihydrogen was found to be endergonic by 4 kcal mol À1 which rules out a redox-based cyclometallation route for uranium.

The synthesis and characterization of the first unsupported Ga-Y bond in [Y{Ga(NArCH)(2)}{C(PPh(2)NSiMe(3))(2)}(THF)(2)] (Ar = 2,6-diisopropylphenyl) is described; structural and computational analyses are consistent with a highly polarized covalent Ga-Y bond.

Four new uranium-ruthenium complexes, [(Tren(TMS))URu(η(5)-C(5)H(5))(CO)(2)] (9), [(Tren(DMSB))URu(η(5)-C(5)H(5))(CO)(2)] (10), [(Ts(Tolyl))(THF)URu(η(5)-C(5)H(5))(CO)(2)] (11), and [(Ts(Xylyl))(THF)URu(η(5)-C(5)H(5))(CO)(2)] (12) [Tren(TMS)=N(CH(2)CH(2)NSiMe(3))(3); Tren(DMSB)=N(CH(2)CH(2)NSiMe(2)tBu)(3)]; Ts(Tolyl)=HC(SiMe(2)NC(6)H(4)-4-Me)(3); Ts(Xylyl)=HC(SiMe(2)NC(6)H(3)-3,5-Me(2))(3)], were prepared by a salt-elimination strategy. Structural, spectroscopic, and computational analyses of 9-12 shows: i) the formation of unsupported uranium-ruthenium bonds with no isocarbonyl linkages in the solid state; ii) ruthenium-carbonyl backbonding in the [Ru(η(5)-C(5)H(5))(CO)(2)](-) ions that is tempered by polarization of charge within the ruthenium fragments towards uranium; iii) closed-shell uranium-ruthenium interactions that can be classified as predominantly ionic with little covalent character. Comparison of the calculated U-Ru bond interaction energies (BIEs) of 9-12 with the BIE of [(η(5)-C(5)H(5))(3)URu(η(5)-C(5)H(5))(CO)(2)], for which an experimentally determined U-Ru bond disruption enthalpy (BDE) has been reported, suggests BDEs of approximately 150 kJ mol(-1) for 9-12.